Germanium FinFETs with Metal Gates and Stressors

ABSTRACT

An integrated circuit structure includes an n-type fin field effect transistor (FinFET) and a p-type FinFET. The n-type FinFET includes a first germanium fin over a substrate; a first gate dielectric on a top surface and sidewalls of the first germanium fin; and a first gate electrode on the first gate dielectric. The p-type FinFET includes a second germanium fin over the substrate; a second gate dielectric on a top surface and sidewalls of the second germanium fin; and a second gate electrode on the second gate dielectric. The first gate electrode and the second gate electrode are formed of a same material having a work function close to an intrinsic energy level of germanium.

This application is a continuation of U.S. patent application Ser. No.12/831,903, entitled “Germanium FinFETs with metal Gates and Stressors,”filed Jul. 7, 2010, which application claims the benefit of U.S.Provisional Application No. 61/245,547, filed on Sep. 24, 2009, andentitled “Germanium FinFETs with Metal Gates and Stressors,” whichapplications are hereby incorporated herein by reference.

TECHNICAL FIELD

This application relates generally to integrated circuit structures, andmore particularly to the structures of fin field effect transistors(FinFETs) and the methods of forming the same.

BACKGROUND

The speeds of metal-oxide-semiconductor (MOS) transistors are closelyrelated to the drive currents of the MOS transistors, which drivecurrents are further closely related to the mobility of charges. Forexample, NMOS transistors have high drive currents when the electronmobility in their channel regions is high, while PMOS transistors havehigh drive currents when the hole mobility in their channel regions ishigh.

Germanium is a commonly known semiconductor material. The electronmobility and hole mobility of germanium are greater (2.6 times and 4times, respectively) than that of silicon, which is the most commonlyused semiconductor material in the formation of integrated circuits.Hence, germanium is an excellent material for forming integratedcircuits. An additional advantageous feature of germanium is thatgermanium's hole and electron motilities have a greater stresssensitivity than that of silicon. For example, FIG. 1 illustrates thehole mobility of germanium and silicon as a function of uni-axialcompressive stresses. It is noted that with the increase in thecompressive stress, the hole mobility of germanium increases at a fasterrate than silicon, indicating that germanium-based PMOS devices have agreater potential to have high drive currents than silicon-based PMOSdevices. Similarly, FIG. 2 illustrates the electron mobility ofgermanium and silicon as functions of uni-axial tensile stresses. It isnoted that with the increase in the tensile stress, the electronmobility of germanium increases at a faster rate than that of silicon,indicating that germanium-based NMOS devices have a greater potential tohave high drive currents than silicon-based NMOS devices.

Germanium, however, also suffers from drawbacks. The bandgap ofgermanium is 0.66 eV, which is smaller than the bandgap of silicon (1.12eV). This means that the substrate leakage currents of germanium-basedMOS devices are high. In addition, the dielectric constant of germaniumis 16, and is greater than the dielectric constant of silicon (11.9).Accordingly, the drain-induced barrier lowering (DIBL) ofgermanium-based MOS devices is also higher than that of silicon-basedMOS devices.

SUMMARY

In accordance with one aspect of the embodiment, an integrated circuitstructure includes an n-type fin field effect transistor (FinFET) and ap-type FinFET. The n-type FinFET includes a first germanium fin over asubstrate; a first gate dielectric on a top surface and sidewalls of thefirst germanium fin; and a first gate electrode on the first gatedielectric. The p-type FinFET includes a second germanium fin over thesubstrate; a second gate dielectric on a top surface and sidewalls ofthe second germanium fin; and a second gate electrode on the second gatedielectric. The first gate electrode and the second gate electrode areformed of a same material having a work function close to an intrinsicenergy level of germanium.

Other embodiments are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates the hole mobilities of germanium and silicon asfunctions of uni- axial compressive stresses;

FIG. 2 illustrates the electron mobilities of germanium and silicon asfunctions of uni-axial tensile stresses;

FIGS. 3 through 9 are perspective views and cross-sectional views ofintermediate stages in the manufacturing of germanium-based FinFETs inaccordance with an embodiment;

FIGS. 10-12 are perspective views and a cross-sectional view ofmultiple-fin FinFETs; and

FIG. 13 illustrates the energy bands of germanium.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use theembodiments, and do not limit the scope of the disclosure.

A novel fin field-effect transistor (FinFET) embodiment and the methodof forming the same are presented. The intermediate stages ofmanufacturing the embodiment are illustrated. The variations of theembodiment are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

Referring to FIG. 3, an integrated circuit structure is formed. Theintegrated circuit structure includes substrate 20, which may be asilicon substrate, a germanium substrate, or a substrate formed of othersemiconductor materials. Substrate 20 may be doped with a p-type or ann-type impurity. Isolation regions such as shallow trench isolation(STI) regions 22 may be formed in or over substrate 20. Germanium fins124 and 224 are formed above the top surfaces of STI regions 22. In anexemplary embodiment, germanium fins 124 and 224 are formed by recessingtop portions of substrate 20 between neighboring STI regions 22 to formrecesses, and re-growing germanium in the recesses. Top portions of STIregions 22 may then be removed, while bottom portions of STI regions 22are not removed, so that the top portion of the re-grown germaniumbetween neighboring STI regions 22 becomes germanium fins. Germaniumfins 124 and 224 may have a germanium atomic percentage greater thanabout 50 percent, for example. In an embodiment, fins 124 and 224 areformed of pure germanium. In alternative embodiments, fins 124 and 224are formed of silicon germanium.

Germanium fins 124 and 224 may have channel dopings. Germanium fin 124may be doped with a p-type impurity such as boron, while germanium fin224 may be doped with an n-type impurity such as phosphorous. Thechannel doping of germanium fins 124 and 224 may be lower than about5E17/cm³, or as low as about 1E17/cm³. In an exemplary embodiment, theaspect ratios of germanium fins 124 and 224 (the ratio of heights H towidths W), may be greater than about 1, or even greater than about 5.Substrate 20 includes a portion in NMOS device region 100 and a portionin PMOS device region 200. Germanium fins 124 and 224 are in NMOS deviceregion 100 and PMOS device region 200, respectively.

Referring to FIG. 4, gate dielectric layer 32 and gate electrode layer34 are deposited in both NMOS device region 100 and PMOS device region200 and over germanium fins 124 and 224. In an embodiment, gatedielectric layer 32 is formed of a high-k dielectric material. Theexemplary high-k materials may have k values greater than about 4.0, oreven greater than about 7.0, and may include aluminum-containingdielectrics such as Al₂O₃, HfAlO, HfAlON, AlZrO, Hf-containing materialssuch as HfO₂, HfSiO_(x), HfAlO_(x), HfZrSiO_(x), HfSiON, and/or othermaterials such as LaAlO₃ and ZrO₂.

Gate electrode layer 34 is formed on gate dielectric layer 32, and maycomprise metal. Gate electrode layer 34 may have a work function closeto an intrinsic level (a middle level, which is about 4.33 eV) of theconduction band of germanium (4 eV) and the valance band of germanium(4.66 eV). In an embodiment, the work function of gate electrode layer34 is between about 4.15 eV and about 4.5 eV, or even between about 4.25eV and about 4.4 eV. Exemplary materials of gate electrode layer 34include Ti_(x)N_(y), Ta_(x)N_(y), Al, Ta_(x)C_(y), Pt, multi-layersthereof, and combinations thereof, with x and y being positive values.

Gate electrode layer 34 and gate dielectric layer 32 are then patternedto form gate stacks, as is shown in FIG. 5. The gate stack in NMOSdevice region 100 includes gate electrode 134 and gate dielectric 132.The gate stack in PMOS device region 200 includes gate electrode 234 andgate dielectric 232. Each of germanium fins 124 and 224 thus hasportions that are uncovered by the gate stacks.

Referring to FIG. 6, gate spacers 136 and 236 may be formed. The exposedportions of germanium fins 124 and 224 not covered by gate dielectrics132 and 232, gate electrodes 134 and 234, and gate spacers 136 and 236are then removed (recessed), while the covered portion of germanium fins124 and 224 are not removed. The removal may be performed by a dry etch.The spaces left by the removed portions of fins 124 and 224 are referredto as recesses 140 and 240, respectively, hereinafter. Recesses 140 and240 may have bottoms level with top surfaces 35 of STI regions 22.Alternatively, the bottoms of recesses 140 and 240 may be lower than topsurfaces 35 of STI regions 22, as illustrated in FIGS. 6.

FIG. 7 (and subsequent FIGS. 8 and 9) illustrates a cross-sectional viewof the structure shown in FIG. 6, wherein the cross-sectional view ofNMOS device region 100 is obtained in a vertical plane crossing line 7-7in FIG. 6, while the cross-sectional view of PMOS device region 200 isobtained in a vertical plane crossing line 7′-7′ in FIG. 6. It is notedthat although FIG. 7 and subsequent FIGS. 8 and 9 illustrate that thecross-sectional views of NMOS device region 100 and PMOS devices 200 arein a same plane, they may actually be in different planes.

Next, as shown in FIG. 8, PMOS region 200 is covered, for example, byphoto resist 241, and source and drain (referred to as source/drainhereinafter) regions 142 are epitaxially grown in recesses 140 byselective epitaxial growth (SEG). Source/drain regions 142 are alsoalternatively referred to as source/drain stressors 142, and may have alattice constant smaller than the lattice constant of germanium fin 124.In an exemplary embodiment, source/drain regions 142 comprise SiGe, andare formed using plasma enhanced chemical vapor deposition (PECVD), orother commonly used methods. The precursors may include Si-containinggases such as SiH₄ and Ge-containing gases such as GeH₄, and the partialpressures of the Si-containing gases and Ge-containing gases areadjusted to modify the atomic ratio of germanium to silicon. In anembodiment, the resulting source/drain regions 142 include between about20 and about 60 atomic percent silicon. In alternative embodiments,source/drain region 142 may be formed of silicon carbon (SiC) or siliconwith no carbon and/or germanium added. N-type impurities, such asphosphorous and/or arsenic, may be in-situ doped when the epitaxialgrowth proceeds. With the lattice constant of source/drain region 142being smaller than that of germanium fin 124, source/drain regions 142apply a tensile stress to germanium fin 124, which forms the channelregion of the resulting n-type FinFET 150. After the epitaxial growth ofsource/drain regions 142, photo resistor 241 is removed.

Referring to FIG. 9, NMOS device region 100 is covered, for example, byphoto resist 141. Source/drains regions 242, which may also be referredto as a source/drain stressors 242, are epitaxially grown in recesses240. Source/drain regions 242 may have a lattice constant greater thanthe lattice constant of germanium fin 224. Again, source/drain regions242 may be formed using PECVD. In an embodiment, source/drain regions242 comprise GeSn. In alternative embodiments, source/drain region 242may be formed of compound semiconductor materials comprising group IIIand group V materials (referred to as III-V semiconductor materialshereinafter), such as InGaAs, InP, GaSb, InAs, AlSb, InSb, and the like.With the lattice constant of source/drain region 242 being greater thanthat of germanium fin 224, source/drain regions 242 apply a compressivestress to germanium fin 224, which forms the channel region of theresulting PMOS FinFET 250. After the epitaxial growth of source/drainregions 242, photo resistor 141 is removed.

During the epitaxial process for forming source/drain regions 142 and242, n-type impurities (such as phosphorous) and p-type impurities (suchas boron), respectively, may be doped with the proceeding epitaxialprocesses. The impurity concentration may be between about 5×10²⁰/cm³and about 1×10²¹/cm³. In alternative embodiments, no p-type and n-typeimpurities are doped, while the doping of source/drain regions 142 and242 are performed in implantation steps after the formation ofsource/drain regions 142 and 242.

Next, silicide/germanide regions (not shown) may be formed onsource/drain regions 142 and 242 by reacting source/drain regions 142and 242 with a metal(s) to reduce the contact resistances. The formationdetails of silicide/germanide regions are known in the art, and henceare not repeated herein. Through the above-discussed process steps,n-type FinFET 150 and PMOS FinFET 250 are formed.

In the above-discussed embodiments, single-fin FinFETs were discussed.Alternatively, the concept of the disclosure may be applied to multi-finFinFETs. FIGS. 10 through 12 illustrate a cross-sectional view andperspective views of multi-fin FinFETs. Unless specified otherwise, likereference numerals are used to represent like elements. The materials ofthe elements shown in FIGS. 10-12 are thus not repeated herein. FIG. 10illustrates a cross sectional view of an integrated circuit includingn-type FinFET 150, PMOS FinFET 250, and dummy fin structures 350,including dummy fins 324, which are formed on substrate 320. Substrate320 may be a germanium substrate or a silicon substrate. N-type FinFET150 is formed on a p-well, and includes multiple germanium fins 124.Gate electrode 134 is formed over multiple germanium fins 124, so thatmultiple germanium fins 124 become the fins of a single n-type FinFET150. Gate dielectrics 132 are formed between germanium fins 124 and gateelectrode 134. Similarly, PMOS FinFET 250 is formed on an n-well, andincludes multiple germanium fins 224. Gate electrode 234 is formed overmultiple germanium fins 224, so that multiple germanium fins 224 becomethe fins of a single PMOS FinFET 250. Gate dielectrics 232 are formedbetween germanium fins 124 and gate electrode 134. In addition, dummyfins, which are not used in any FinFETs, are also formed to reduce thepattern-loading effect in the formation of germanium fins 124 and 224.

With multiple fins used in a single FinFET, the drive current of theFinFET can be further increased. Since there is a lattice mismatchbetween germanium and silicon, it is easier achieve a high quality (withlower defect density) for a germanium epitaxy layer grown from a finwith a smaller fin width than from a fin with a greater fin width.

FIGS. 11 and 12 illustrate perspective views of multiple FinFETs. Thelike elements in FIG. 11 can be found in FIG. 10. The FinFET may eitherbe n-type FinFET 150 or PMOS FinFET 250, and hence is denoted as150/250. In FIG. 11, source/drain regions (stressors) 142/242 are grownfrom germanium fins 124/224, and are discrete regions. In FIG. 12,source/drain regions (stressors) 142/242 grown from germanium fins124/224 merge with each other.

FIG. 13 illustrates the energy bands of germanium. It is noted thatgermanium has a conduction band Ec equal to 4 eV, a valence band Evequal to 4.66 eV, and an intrinsic level Ei (which is (Ec+Ev)/2) equalto 4.33 eV. Therefore, the intrinsic level Ei and the conduction band Echave an energy difference equal to about 330 mV, and the intrinsic levelEi and the valence band Ev have an energy difference equal to about 330mV. The 330 mV energy difference may be utilized to simplify theformation of metal gates for n-type germanium FinFETs and p-typegermanium FinFETs. Since in germanium FinFETs, the fully depletedchannel results in the reduction in threshold voltages Vt, band-edgework functions are no longer needed. Instead, near-mid-bandgap workfunctions are needed to shift the threshold voltages Vt to target valuesaccurately. Accordingly, for germanium-based FinFETs, with the workfunctions of metal gates of both n-type germanium FinFETs and p-typegermanium FinFETs being close to the intrinsic level of about 4.33 eV,the requirements for optimizing the work functions of n-type FinFETs andp-type FinFETs can both be satisfied, even when a same metallic materialis used forming the gates of n-type FinFETs and p-type FinFETs.

In addition to the above-discussed advantageous features, theembodiments of the disclosure have several other advantageous features.By forming germanium-based FinFETs, the drive currents of n-type FinFETsand p-type FinFETs can be improved due to the high electron and holemobilities of germanium. The leakage currents may also be reduced due tothe reduced junction areas of FinFETs compared to planar MOS devices.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a firstgermanium-containing fin higher than top surfaces of Shallow TrenchIsolation (STI) regions, wherein the STI regions are on opposite sidesof the first germanium-containing fin and extend into a siliconsubstrate, and the first germanium-containing fin has a first germaniumatomic percentage; forming a first gate stack on a middle portion of thefirst germanium-containing fin, with an end portion of the firstgermanium-containing fin exposed; etching the end portion of the firstgermanium-containing fin to form a recess; and re-growing a firstsource/drain region in the recess, wherein the first source/drain regionhas a second germanium atomic percentage lower than the first germaniumatomic percentage.
 2. The method of claim 1, wherein the first gatestack and the first source/drain region are parts of an n-type Fin FieldEffect Transistor (FinFET).
 3. The method of claim 1, wherein the firstsource/drain region extends below the top surfaces of the STI regions tocontact a portion of the silicon substrate, with the portion of thesilicon substrate extending between and at a same level as the STIregions.
 4. The method of claim 1, wherein the firstgermanium-containing fin is free from silicon.
 5. The method of claim 1further comprising forming a p-type Fin Field Effect Transistor (FinFET)comprising: when the first germanium-containing fin is formed,simultaneously forming a second germanium-containing fin over thesilicon substrate.
 6. The method of claim 5 further comprising: forminga second gate stack on a middle portion of the secondgermanium-containing fin, with an end portion of the secondgermanium-containing fin exposed; etching the end portion of the secondgermanium-containing fin to form an additional recess; and re-growing asecond source/drain region in the additional recess, wherein the secondsource/drain region has a third germanium atomic percentage higher thanthe first germanium atomic percentage.
 7. The method of claim 6, whereineach of the forming the first gate stack and the forming the second gatestack comprises: forming a gate dielectric on a top surface andsidewalls of a respective one of the first germanium-containing fin andthe second germanium-containing fin; and forming a gate electrode overthe gate dielectric, wherein the gate electrode of the first gate stackand the gate electrode of the second gate stack are formed of a samematerial having a work function between about 4.25 eV and about 4.4 eV.8. The method of claim 1, wherein the forming the firstgermanium-containing fin comprises: recessing a top portion of thesilicon substrate between the STI regions to form an additional recess;re-growing a germanium-containing semiconductor material in theadditional recess; and recessing the STI regions.
 9. The method of claim1 further comprising forming gate spacers on sidewalls of the firstgermanium-containing fin, wherein in the etching the end portion of thefirst germanium-containing fin, the middle portion of the firstgermanium-containing fin is protected by the first gate stack and thegate spacers.
 10. A method comprising: forming Shallow Trench Isolation(STI) regions extending into a substrate, with the substrate comprisingsilicon; forming a first germanium fin and a second germanium fin higherthan portions of the STI regions on opposite sides of respective ones ofthe first and the second germanium fins, wherein both the first and thesecond germanium fins have a first germanium atomic percentage; formingan n-type Fin Field Effect Transistor (FinFET) comprising: forming afirst gate dielectric on a top surface and sidewalls of the firstgermanium fin; forming a first gate electrode over the first gatedielectric; etching a portion of the first germanium fin to form a firstrecess; and growing a first source/drain region comprising germanium inthe first recess, with the first source/drain region having a secondgermanium atomic percentage lower than the first germanium atomicpercentage, and the first source/drain region extends below top surfacesof the STI regions to contact the substrate; and forming a p-type FinFETcomprising: forming a second gate dielectric on a top surface andsidewalls of the second germanium fin; forming a second gate electrodeover the second gate dielectric; etching a portion of the secondgermanium fin to form a second recess; and growing a second source/drainregion comprising germanium in the second recess, with the secondsource/drain region having a third germanium atomic percentage higherthan the first germanium atomic percentage, wherein the secondsource/drain region extends below the top surfaces of the STI regions tocontact the substrate, and wherein the first gate electrode and thesecond gate electrode are formed of a same material having a workfunction between 4.25 eV and 4.4 eV.
 11. The method of claim 10, whereinthe first and the second germanium fins are free from silicon.
 12. Themethod of claim 10, wherein the first gate electrode and the second gateelectrode are formed simultaneously, and are formed of a same metallicmaterial.
 13. The method of claim 10, wherein the growing the secondsource/drain region comprises growing germanium tin (GeSn).
 14. Themethod of claim 10 further comprising: forming a third germanium finunderlying the first gate electrode, wherein the third germanium fin isphysically separated from, and electrically connected to, the firstgermanium fin; and forming a fourth germanium fin underlying the secondgate electrode, wherein the fourth germanium fin is physically separatedfrom, and electrically connected to, the second germanium fin.
 15. Amethod comprising: forming Shallow Trench Isolation (STI) regionsextending into a substrate; forming a p-type Fin Field Effect Transistor(FinFET) comprising: forming a first germanium fin over the substrate,wherein the first germanium fin is a germanium fin without being dopedwith silicon; forming a first gate dielectric on a top surface andsidewalls of the first germanium fin; forming a first metal gate overthe first gate dielectric; and growing a first source/drain regionadjacent to the first metal gate; and forming an n-type FinFETcomprising: forming a second germanium fin over the substrate, whereinthe second germanium fin is a germanium fin without being doped withsilicon; forming a second gate dielectric on a top surface and sidewallsof the second germanium fin; forming a second metal gate over the secondgate dielectric, wherein the first metal gate and the second metal gateare formed simultaneously; and growing a second source/drain regionadjacent to the second metal gate, wherein the second source/drainregion comprises silicon germanium, with a germanium atomic percentagein the second source/drain region being lower than a germanium atomicpercentage in the second germanium fin, wherein the second source/drainregion extends below top surfaces of the STI regions to contact thesubstrate.
 16. The method of claim 15, wherein the first germanium finand the second germanium fin are formed simultaneously in a sameepitaxial growth.
 17. The method of claim 15, wherein the forming thefirst source/drain region of the p-type FinFET comprises growing a III-Vcompound semiconductor material.
 18. The method of claim 15, wherein theforming the first source/drain region of the p-type FinFET comprisesgrowing germanium tin (GeSn).
 19. The method of claim 15, wherein thefirst metal gate and the second metal gate are formed of a same materialhaving a work function between 4.25 eV and 4.4 eV.
 20. The method ofclaim 15, wherein the first germanium fin and the second germanium finare free from silicon.